1. Field of the Invention
The present invention relates to an observation apparatus for allowing an operator to observe an object, usable with, for example, a mask aligner for manufacturing semiconductor circuits wherein a mask and wafer are to be aligned, an inspection apparatus for inspecting semiconductor devices and a microscope, or the like.
2. Description of the Prior Art
As an example of apparatus wherein two objects are automatically aligned in a predetermined positional relationship, an automatic alignment apparatus is used in manufacturing semiconductor integrated circuits. U.S. Pat. No. 4,251,129 which has been assigned to the Assignee of the present application, shows an example of such an apparatus. This is shown in FIG. 1A for the sake of explanation, wherein alignment marks on a mask 1 and a wafer 2 which are the objects to be aligned are scanned by a light spot formed by a laser beam. The electric signals produced in response to the beam reflected by the marks, more particularly, the time intervals between the signals, are used to discriminate the alignment. The apparatus contains, in addition to such a photoelectric detection system, an optical system for allowing the operator to observe the objects with his or her eyes. The observation optical system not only allows the observation of the patterns, but also plays an important role upon the scanning operation which is inevitable for the initial alignment of the mask 1.
The apparatus of FIG. 1A includes, in addition to the laser beam source 11 for producing a beam for the photoelectric detection system, a light source for the observation optical system, which is used exclusively for the observation. For this reason, the apparatus optical elements which match the wavelength of the laser beam and also that of the observation light source. The apparatus will be explained in further detail. Along the path of the laser beam L emitted from the laser source 11, for example, a He-Ne laser, there are provided a condenser lens 12 and a polygonal mirror 13 in this order. Along the optical axis X of the laser beam L deflected by the polygonal mirror 13, there are provided an f-.theta. lens 14, dichroic beam splitter 15 for dividing the reflected observation light out to the observation optical system, a field lens 16, a polarization beam splitter 17 for dividing the reflected alignment beam out to the photoelectric detection optical system, a relay lens 18, a beam splitter 19 for introducing the observation light into the main optical system, an aperture stop 20 and an adjacent lens 21, in the order named. At the position where the laser beam L is imaged, there is the mask 1. A pattern of the mask 1 is imaged, through the imaging optical system 22, on the wafer 2 which is placed at a position optically conjugate with the mask 1. Within the imaging optical system 22 is provided a .lambda./4 plate 23 which changes the state of polarization of the beam when it passes therethrough.
Along the optical axis for the reflected beam divided out by the polarization beam splitter 17, there are an imaging lens 30, a wavelength filter 31, a partial blocking plate 32, a condenser lens and a photoelectric transducer 34. The observation light from the light source 40 reaches, through a condenser lens 41, a filter 42 and a polarization plate 43, the beam splitter 19, by which it is reflected toward the objective lens 21. Along the optical axis for the observation light divided out by the dichroic beam splitter 15, there are an imaging lens 50, a filter 51 and an erector 52 to allow an operator to observe the object illuminated by the observation light.
In operation, the laser beam L emitted from the laser source 11, is condensed at the position, shown by reference character a, by a condenser lens 12, then it is incident on the polygonal mirror 13 whereby it is reflected at a right angle and also scanningly deflected in a plane of the rotation thereof. The laser beam L passes through the f-.theta. lens 14, the beam splitter 15 and the field lens 16, and it is again condensed at the position shown by reference numeral b, and then passed through the polarization beam splitter 17, the relay lens 18 and the beam splitter 19. The beam L further goes through the aperture stop 20 so that the principal ray of the beam L passes through the focal point c of the objective lens 21 which is located at the center of the aperture 20. Then, the beam is incident on the objective lens 21. Since the principal ray passes through the focal point c of the objective lens 21, it becomes parallel to the optical axis after it passes therethrough. The principal ray, therefore, is incident on the mask 1 perpendicularly thereto as linearly polarized light. The beam is then condensed on the wafer 2 by the imaging optical system 22, since the wafer 2 is located on the image plane of the optical system 22. At this time, the laser beam which is linearly polarized is converted to a circularly polarized beam by the .lambda./4 plate 23. The laser beam L incident on the mask 1 and the wafer 2 is imaged as a spot thereon by the objective lens 21, and those spots scan the mask 1 and the wafer 2 in a plane including the face of FIG. 1A in accordance with the rotation of the polygonal mirror 13. The beam L reflected by the mask 1 and the wafer 2 passes back through the .lambda./4 plate 23 so that the direction of polarization is changed by 90 degrees.
The beam reflected by a flat surface (specular surface) of the mask 1 and the wafer 2, that is, the surface portions other than the mark portions, is not scattered so that the reflected beam traces back on the oncoming path. The beam passes the entrance pupil at focal point c and the point adjacent it, and goes back to the beam splitter 17 via a relay lens 18. When, however, the scanning spot is incident on a non-specular part of the surface, that is, the alignment marks M1 and M2 as shown in FIG. 1B, the laser beam is reflected and scattered by the edges of the marks. The scattered light does not necessarily trace back. As shown by the dotted lines, the scattered light, after passed back through the objective lens 21, does not necessarily passes through the center of the entrance pupil, i.e., the focal point c, and goes through the marginal area thereof. This means that the scattered light and the non-scattered light are spatially separated at the pupil.
The non-scattered light, that is the light reflected by the specular surfaces of the mask 1 and the wafer 2, is directed to the beam splitter 17 through the relay lens 18 which is effective to converge the beam so as to image the beam at the point b. The beam splitter 17 divides the incident beam into a beam reflected toward the photoelectric detection system and the other beam passing therethrough toward the polygonal mirror 13.
The non-scattered light deflected toward the photoelectric detection system passes through the imaging lens 30 which is effective to establish the optical conjugate relation between the focal point b of the relay lens 18 and the partial light blocking plate 32, and through the wavelength filter 31 which allows the light for the photoelectric detection to pass through but blocks the light coming from the observation light source 40, and then images on the partial light blocking plate 32 at its center. The blocking plate 32 made of a transparent glass has its central area patterned or covered by a light blocking material. Therefore, the non-scattered light does not go beyond the blocking plate 32 to the condenser lens 33 and the photoelectric transducer 34. The scattered beam, however, does not necessarily condense on the center of the blocking plate 32 (even if it goes back along the path generally similarly to the non-scattered light), so that the scattered light reaches the photoelectric transducer 34. Thus, the photoelectric transducer 34 receives light and produces electric signal, only when the scanning laser beam illuminates the alignment mark.
The beam emitted by the observation light source 40 does not entirely pass through the polarization plate 43, but only the component of the polarization in the direction perpendicular to the polarization direction of the laser beam can be transmitted. The passed beam is then is reflected to the objective lens 21 by the beam splitter 19. The reflected beam, similarly to the laser beam, is reflected by the wafer 2, and the polarization direction is changed by 90 degrees by the .lambda./4 plate 23 and transmitted through the objective lens 21 toward the polygonal mirror 13, and passed through the beam splitter 19 and polarization beam splitter 17, and then reflected by the dichroic beam splitter 15. The erector 52 forms an erected image to allow observation by the human eyes. The polarization beam splitter 17 prevents the beam directly reflected by the mask 1 and incident on the objective lens 21 from being reflected toward the erector 52 so as to introduce only the beam which has reached the wafer 2. Because of this, the image can be observed as a sharp image without flare, which may otherwise be created by the light reflected by the mask 1.
In order to achieve the effective reception of the laser beam L by the photoelectric transducer 34 and the effective observation with the observation light from the observation light source 40, the characteristics of the optical element must be determined very carefully. For example, a linearly polarized laser beam is used, in which the direction of polarization is made parallel with the face of FIG. 1. That is, the laser beam source is so oriented. On the other hand, for the observation optical system, the part of the laser beam L polarized in the direction perpendicular to the face of FIG. 1 is used. The polarization plate 43 is so disposed. For the wavelength of the laser beam used, the dichroic beam splitter 15 reflects the P-polarized component, while the beam splitter 19 transmits the P-polarized component, further the polarization beam splitter 17 transmits the P-polarized component and reflects the S-polarized component.
Actually, however, the dichroic coating has a limit determined by the refraction index of the evaporated material, so that it is difficult to achieve the above described characteristics. For example, when a parallel plane type beam splitter is used as the beam splitter 19 instead of the prism type beam splitter, it is difficult to provide the above described characteristics for the P-polarization component and for the S-polarization component. The particular cause for this problem is the fact that the wavelength of the beam from the laser source 11 and that of the light from the observation light source 40 are different. The characteristics of the beam splitter 15 must be considered with the view to plural kinds of wavelength. This is a limit to the latitude of the design of the apparatus. In addition, two light sources which are used make the structure complicated.